The present invention relates in general to the field of lasers, and specifically to laser diodes that contain an integral photodiode for monitoring and controlling laser intensity.
Laser diodes have been manufactured for years and most contain a silicon photodiode inside a laser can. This photodiode (PD) is used to monitor the power of the laser and can be connected to a feedback circuit to coarsely control the laser intensity as the laser heats up.
FIG. 1 shows a semiconductor laser or laser diode 9 with a monitor photodiode 11 inside a sealed can defined by a cap 15. Monitor photodiode 11 is attached to a stem 21. A semiconductor laser element or laser chip 17 is mounted on a heat sink 19, which is attached to stem 21. This configuration is covered by a container or cap 15, which has window 23 which allows passage of laser light.
As also shown in FIG. 1, semiconductor laser 9 includes a terminal 25 which provides a connection to a control circuit 27. Terminal 25 includes a first line 25a which leads to laser element 17, a second line 25b which leads to sensor or photodiode 11, and a third line 25c which is a ground wire. Control circuit 27 energizes semiconductor laser element 17 in a known manner so as to emit a laser beam 100 with a light power output P0 through window 23. As further illustrated in FIG. 1, photodiode 11 is fixed to stem 21 with its light receiving surface 11a facing laser element 17. Laser beam 100 with light power output P0 is emitted from laser element 17 through window 23 and at the same time, a monitor beam 110 with a light power output of about 3% of P0 is emitted from laser element 17 toward photodiode 11. Monitor beam 110 incident on the monitor photodiode 11 generates a monitor signal. This monitor signal is fed back to control circuit 27 for driving laser element 17 so that the power output P0 of the laser beam 100 emitted from the laser element 17 is maintained in a stable state. A typical example of a control circuit as described above is shown in, for example, U.S. Pat. No. 5,067,117.
In early designs, laser diodes were made in a wafer and laser elements 17 were scribed and cleaved to form front and back facets. These facets were not coated but had a reflectivity of 30% to 35% because of the high index of the semiconductor (GaAs, AlGaAs) material. This reflectivity was sufficient to attain a round trip gain of greater than unity to establish laser action. A problem with this design is that half of the light came out the front facet of the laser element, and the other half came out the back to strike the photodiode. An example of such a laser is the Hitachi 8312. In such a laser, the photodiode output appeared to track reasonably well with the laser output and a measurement of photodiode current vs. laser power was very linear.
In an effort to provide more power, laser manufacturers started to introduce lasers with dielectric mirrors on the back facet and modest anti-reflection (AR) coatings on the front facet. The mirror directed most of the light out the front of the laser, while allowing a small amount to leak through the back facet to the photodiode which was used to monitor and control laser power. FIG. 2 shows a conventional laser element or laser chip design 32 with a multilayer mirror on the back facet. The structure of FIG. 2 comprises a laser element 34 with a rear facet multilayer mirror 36 comprised of alternating amorphous silicon and aluminum oxide layers; and a front facet AR (aluminum oxide) coating 38. A rated laser power P0 (40), is emitted from the front facet, and a small fraction, P0/30 (42) is emitted from the rear facet to the photodiode.
The Hitachi 8314 laser has basically the same structure as the Hitachi 8312 laser, except that a multilayer mirror and an AR coating were applied as shown in FIG. 2. The output power available was effectively doubled, but a measurement of photodiode current vs. laser power showed some non-linearity even if the laser stayed in a single longitudinal mode. This led to problems with respect to the photodiode current not being well correlated to the laser front facet power. At times it would be well correlated, and then behave erratically and unpredictably. In order to achieve a tight control of laser power, some laser arrangements included a complicated means of sampling the front facet beam with a prism or window to provide a well correlated signal for power control (see, for example, U.S. Pat. Nos. 5,067,117, 4,989,198, and 5,363,363).
Conventional multilayer periodic mirror systems are made of quarter wave thick layers of alternating high index and low index films. Versions of this structure are used by laser manufacturers to make high reflectivity mirrors for the back facet of the laser as shown in FIG. 2. Such structures can be fairly uniform in reflectivity for a large range of wavelengths. However, it is the transmittance of coherent light in such structures that is the cause of the erratic behavior of the back facet photodiode output. When the laser heats up causing mode drift or mode hops, the wavelength changes, slowly and predictably in the first case and rapidly and erratically in the second case. When the laser is mode hopping, the correlation between the laser power and the photodiode current tends to be poor.
A back facet mirror made up of multiple quarter wavelength layers as described above can act as a Fabry-Perot etalon or an optical bandpass filter. The undesirable Fabry-Perot effect of the multilayer mirror can be better understood by examining the effect of a more typical Fabry-Perot etalon such as a glass plate. FIG. 3 shows such an etalon which is basically a parallel plane glass plate 50 in a path of a laser beam 52 emitted by a laser 53. Laser beam 52 passes through glass plate 50 and strikes a photodetector 54. The power transmitted by glass plate 50 and measured by detector 54 is a function of the wavelength of laser 53, due to constructive and destructive interference within the glass plate 50 from reflections from the two surfaces of glass plate 50.
FIG. 4 illustrates a graph which shows the transmittance of glass plate 50 as a function of the wavelength and as a function of the reflectivity of the two surfaces of glass plate 50, and is effectively the light level at detector 54 of FIG. 3. As the reflectivity of the surfaces of glass plate 50 increases, the transmitted peaks narrow and the variation between the maximum and minimum level of light transmitted increases. In other words, the Fabry-Perot effect increases with reflectivity. Even a low reflectivity of about 4% has a noticeable effect. As the laser mode hops and changes wavelength, the light transmitted through the back facet mirror can vary even though the laser output from the front facet remains constant. There are three approaches to decrease the Fabry-Perot effect: (1) decrease the reflectivity of the surfaces of the glass plate, (2) add neutral density between the two surfaces of the glass plate to reduce the interference, and (3) tilt the glass plate in the beam such that the transmitted beam and the reflected beam do not overlap each other. In a laser diode, the last two approaches (2 and 3) are not easy to implement. The first approach (1) which may involve decreasing the reflectivity to as low as 4% will still not eliminate the Fabry-Perot effect, but it will inhibit the laser from lasing. Thus, the original cleaved, uncoated facet is preferable to a low reflectivity multilayer mirror.
In view of the above discussion, it can now be seen why the back facet diode current sometimes tracks the laser output very well and sometimes behaves very erratically. As the laser changes wavelength, the light transmitted to the back facet photodiode can vary substantially even though the laser output power has not changed.
For product designs where the back facet photodiode provides inadequate control, complicated front facet pickoff methods such as shown in U.S. Pat. No. 5,067,117 appear to be the typical solution. Methods of fabricating lasers to provide a monolithic photodiode are described in U.S. Pat. Nos. 5,258,991; 5,568,502; 4,769,342; and 5,438,208. Such monolithic detectors are directed at fabricating the detector at the same time as the laser, thereby reducing time consuming and costly assembly of the photodetector inside the laser can. U.S. Pat. No. 5,068,868 shows a front facet metallic mirror and a dielectric back facet mirror. In U.S. Pat. No. 5,068,868, no mention is made of a photodiode to monitor the power of the laser. Also, a typical monitor photodiode is difficult to implement in a vertical cavity laser due to its structure.
The present invention provides for a novel system, method and semi-conductor laser which overcome the drawbacks noted above. The system, method and semi-conductor laser of the present invention include a monitor photodiode that monitors and controls laser intensity. A laser element or laser chip of the laser of the present invention includes a back facet that comprises a metallic coating. The metallic coating overcomes the drawbacks of dielectric mirror surfaces and permits sufficient transmittance and constant reflectivity. This provides for an adequate signal to the monitor photodiode, and at the same time provides for an adequate and uniform reflectivity so as to maintain constant laser power output levels.
The present invention relates to a system for controlling laser power output levels which comprises a laser; a control circuit for energizing the laser; and a sensor for measuring a laser power output from a back facet of the laser. The sensor provides feedback to the control circuit for maintaining laser power output levels. The back facet of the laser is covered with a coating and transmission and reflection characteristics of the coating are relatively insensitive to minor changes in wavelength of the laser.
The present invention further relates to a method of controlling laser power output levels which comprises the steps of forming front and back facets on a laser element of a laser; covering the back facet of the laser element with a coating having transmission and reflection characteristics that are relatively insensitive to minor changes in wavelength of the laser; and positioning the laser element on the laser so that the back facet faces a sensor on the laser. The sensor measures power output from the back facet.
The present invention also relates to a laser which comprises a container having an aperture; a laser element positioned in the container and having front and back facets, with the front facet of the laser element facing the aperture; and a sensor having a light receiving surface which faces the back facet of the laser element. The sensor measures a power output from the back facet of the laser element. The back facet is covered with a coating layer having transmission and reflection characteristics which are relatively insensitive to minor changes in wavelength of the laser.